Refractory castables are widely used in industrial furnaces and kilns in metallurgy, cement, power, and petrochemical industries due to their convenient construction, good integrity, and excellent high-temperature resistance. However, even the highest quality castables can suffer from problems such as spalling, slag seepage, cracking, and even bursting if construction is not carried out correctly. Therefore, mastering the three core processes of mixing, casting, and curing is crucial for extending the life of furnace linings. This article focuses on the construction process, explaining in detail the correct construction methods and precautions for refractory castables.

I. Castable Mixing Methods

The quality of mixing determines the fluidity, density, and final strength of the castable.

1. Selection of Mixing Equipment

Vertical or horizontal forced mixers are recommended.

Manual mixing or free-type (drum) mixers are prohibited, as they easily cause material segregation.

2. Water Addition Control

The amount of water added to the castable must strictly follow the manufacturer's standards, generally 4%–10%. Excessive water addition: Reduced strength, increased shrinkage, and increased susceptibility to cracking.

Insufficient water addition: Poor fluidity, difficulty in molding, and voids.

Note: Adding water arbitrarily based on experience is strictly prohibited.

3. Mixing Steps

Dry Mixing: Pour aggregates, fine powder, and additives into the mixer and dry mix for 2-3 minutes to ensure even distribution of the powder.

Wet Mixing: Add water while mixing, ensuring that the water penetrates evenly into the matrix and aggregates.

Total mixing time is generally 3-5 minutes. The material should be fluid, smooth, free of lumps, and uniformly distributed.

4. Mixing Site Requirements

The mixing area should be far from dust and kept clean.

Prevent exposure to direct sunlight or strong winds that could cause moisture loss.

The mixed castable should be used within 30 minutes; secondary mixing with water is prohibited.

II. Casting Methods for Refractory Castables

The casting process directly affects the strength, density, and erosion resistance of the furnace lining structure.

1. Template and Substrate Treatment

The template should be clean, sturdy, and waterproof.

To facilitate demolding, a suitable amount of release agent can be applied (avoiding interference with the bonding of the castable refractory).

The substrate should be thoroughly cleaned, removing rust, impurities, and loose dust.

For curved surfaces and special areas, wooden or steel templates can be used.

2. Pouring Method

Layered pouring is typically used, with each layer approximately 200-300mm thick.

Layered pouring helps with vibration compaction and prevents the formation of "cold joints."

Large-area pouring should be continuous, with intervals not exceeding 30 minutes.

3. Vibration Compaction Process

Vibration is a crucial step in castable refractory construction.

Key points for using vibrators:

Insertation method: Quick insertion, slow withdrawal to avoid creating voids.

Vibration time: Generally 10-20 seconds; prolonged vibration at the same location is not recommended.

Vibration Density: Insertion point spacing 250-300mm to ensure complete compaction.

Precautions:

Excessive vibration time may cause aggregate settling and segregation.

Corners and dead corners can be compacted using a small vibrator or manually.

4. Surface Treatment

After casting, the surface should be smoothed, free from defects such as cracks, honeycombing, and voids.

If the ambient temperature is above 30℃, cover with a film to prevent rapid water loss.

III. Curing Methods for Castable Refractories

Curing is a crucial process for castable refractories to gain strength and is the most easily overlooked yet critical aspect of construction quality.

1. Natural Curing Stage (24-48 hours)

Curing Requirements:

Cover immediately after construction to prevent moisture evaporation.

Maintain a temperature of 10℃-30℃; low temperatures require insulation measures.

Strictly prohibit exposure to direct sunlight, wind, and rain.

During the curing period, avoid stepping on or impacting the cast body to ensure its stability.

4. Furnace Drying and Preparing Stage

Furnace drying is a crucial step in refractory castable construction, used to remove free water and chemically bound water to prevent cracking.

Furnace drying principles: low-temperature, slow drying, uniform heating.

Typical heating curves for reference:

100℃, hold for 3-4 hours: remove free water

300℃, hold for 3-6 hours: remove bound water

600-800℃, hold for more than 3 hours: begin sintering

Above 1000℃, heat as required

Rapid heating can lead to cracking and spalling, a major cause of furnace lining damage.

IV. Common Problems and Preventive Measures

1. Cracking of the Castable

Causes: Excessive water addition, improper curing, rapid furnace drying.

Measures: Strictly control water volume, cover and cure, and dry the furnace according to the heating curve.

2. Porosity and Looseness

Causes: Insufficient vibration or under-vibration.

Measures: Layered vibration, reasonable density, and avoid missed vibration.

3. Surface powdering

Causes: Incomplete cement hydration or insufficient curing.

Measures: Extend curing time and avoid excessively low temperatures.

V. Conclusion

The mixing, casting, and curing of refractory castables are the three core aspects affecting the furnace life. Only by achieving:

"Correct formula → Uniform mixing → Vibration compaction → Sufficient curing → Scientific furnace drying"

can the castable structure be guaranteed to be dense, erosion-resistant, and have a long service life, thereby effectively improving the operational safety and economic benefits of the equipment.